CHAP. IX. SENSITIVENESS TO LIGHT. 451 



and of Smilax aspera ; and the stems of some plants 

 which climb by rootlets, as those of the Ivy and Tecoma 

 radicans, are likewise apheliot.ropic, and they thus find 

 a support. The leaves, on the other hand, of most 

 climbing plants are heliotropic ; but we could detect 

 no signs of any such movement in those of Mutisia 

 clematis. 



As heliotropism is so widely prevalent, and as 

 twining plants are distributed throughout the whole 

 vascular series, the apparent absence of any tendency 

 in their stems to bend towards the light, seemed to 

 us so remarkable a fact as to deserve further in- 

 vestigation, for it implies that heliotropism can be 

 readily eliminated. When twining plants are exposed 

 to a lateral light, their stems go on revolving or cir- 

 cumnutatirig about the same spot, without any evident 

 deflection towards the light ; but we thought that 

 we might detect some trace of heliotropism by com- 

 paring the average rate at which the stems moved to 

 and from the light during their successive revolutions.* 

 Three young plants (about a foot in height) of Ipomcea 

 cverulea and four of I. purpurea, growing in separate 

 pots, were placed on a bright day before a north-east 

 window in a room otherwise darkened, with the tips 

 of their revolving stems fronting the window. When 

 the tip of each plant pointed directly from the window, 

 and when again towards it, the times were recorded. 

 This was continued from 6.45 A.M. till a little after 

 '2 r.M. on June 17th. After a few observations we 

 concluded that we could safely estimate the time 



* Some erroneous statements number of observations, for \ve did 

 are unfortunately given on tbis not then know at how unrqual 

 subject, in 'The Movements and a rate the steins and tendrils of 

 Habits of Climbing Plants,' 1875, climbing plants rometimes travel 

 pp. 28, 32, 40, and 53. Conclusions in different parts of the same re- 

 were drawn from an insufficient volution. 



